The sensor in question is an optical, mechanical and electronic device capable of detecting the temperature distribution of an infrared image formed by an objective which operates in the 8-12 micrometer spectral band.
Infrared image sensors are known (e.g. from EP-A-0 367 481), which include: an optical focusing system to focus an image on a matrix of sensitive elements for the detection of said image; a focal plane disposed at the entry of said sensor where an infrared image is formed, a collimator unit, a reimaging optics, and at least one thermal reference on said focal plane, whose image is projected on said matrix (55).
The possibility of detecting the thermal image of a target is dependent on the availability of electro-optical components which are sensitive to such radiation and whose sizes and number of elements are adequate to supply the necessary detail of the image.
At the present time, one-dimensional rows of elements, called "arrays", are used to detect the infra-red image in this spectral band; they only permit the detection of a single row of image elements (pixels) at one time. The whole frame of the image may be reconstructed over a period of time, by moving the array over it by means of an optical scanning system. Usually, there are empty spaces between the detecting elements of the array, necessitating a second micro-scanning movement, called an interlacing movement. This movement takes place at the end of each scanning operation, in a direction orthogonal to it, and causes a displacement of the array equal to half the distance between two adjacent elements. In this way, the whole surface of the image is detected in two scanning operations (half-frames). The assembly of sensitive elements which form the array is contained in a sealed container which, in order to increase the sensitivity of the detectors, is kept at a temperature of approximately -200.degree. C. The cold casing which encloses the array has a circular aperture in front of the array, called a cold filter, through which passes the infrared radiation which forms the thermal image. Said filter forms the entry aperture of all the elements of the array. The image of the aperture diaphragm, called the exit pupil, of the optical system which forms the infrared image is usually conjugate on this filter.
The detection of the infrared image consists in a succession of electrical signals emitted by each detector element, the intensity of these signals being a function of the temperature detected in the image. These signals, when suitably processed electronically, may be used to control a television set which reconstructs a visible representation of the infrared image.
The electronic processing of the signals requires the comparison of these signals with a known temperature reference, in order to calibrate their response on a scale of absolute temperature. Usually, this reference is provided by mechanical elements disposed at the sides of the image on a focal plane inside a telescope which is disposed in front of the image sensor itself. This telescope is designed each time according to the specific application and does not form part of the infrared sensor. Essentially, therefore, the thermal references are outside the sensor and form part of the optical system located in front of the sensor itself.
The thermal reference elements are kept at controlled predetermined temperatures, and are detected together with the image during scanning, by a suitable extension of the path beyond the useful width of the image.
The infrared radiation originating from the object or scene which is observed through a suitable telescope which changes the enlargement with which it is seen by the sensor. As mentioned above, this telescope is not an integral part of the sensor. Its characteristics depend on the specific requirements of the application which uses the sensor. The sensor itself receives the beam re-collimated by the telescope and consists of the scanning devices and the optical system for focusing on the array of sensors. At the present time, automatic focusing systems which keep the image in focus when the ambient temperature varies are disposed in the objective in addition to the thermal references mentioned above. These automatic focusing systems are necessary because the refraction coefficient of the material used for the optical systems in the far infrared (typically germanium) varies considerably with the temperature, as a consequence of the variation of the density of the material.
This gives rise to a number of problems which will be clarified in the following text, and which are overcome by the present invention.